The conventional adhesive tapes in which pressure-sensitive adhesive compositions containing diene-based polymers such as natural rubber and styrenebutadiene copolymer as a base polymer are used are produced by applying an adhesive solution prepared by mixing and dissolving base polymer, tackifier resin, softening agent, stabilizer, filler, pigment and other additives in an organic solvent to a tape substrate, evaporating and removing the organic solvent by passing the coated tape through a drier, and then winding up and cutting the dried tape.
However, in recent years, the possibility of introducing the hot melt production technique into the above production process has been investigated from the standpoints of increased productivity, environmental pollution prevention, safety from fire and so forth, and such technique is now under the commercial utilization in some sections, in particular in the production of pressure-sensitive adhesive tapes for packaging purposes. The base polymers for pressure-sensitive adhesive compositions used in the above technique are mainly an A-B-A type block copolymer. In the prior art, each A in the above copolymers is a polystyrene block and B is a polyisoprene or polybutadiene block, and the above two A blocks have substantially the same number average molecular weight in the range of from 10,000 to 20,000. Pressure-sensitive adhesive compositions using such block copolymers are disclosed in detail, for example, in U.S. Pat. No. 3,239,478.
However, when pressure-sensitive adhesive compositions using the conventional block copolymers are applied as adhesive tapes for packaging there is the disadvantage that the sealing of corrugated cardboard is poor.
The base polymer which has been conventionally used as the base polymer in pressure-sensitive adhesive compositions for the adhesive tapes for packaging is a natural rubber and, even when corrugated cardboard flaps are sealed with such tapes under a light pressure by hand, good corrugated cardboard sealing can be achieved because a sufficient adhering area can be secured and the repulsive force of corrugated cardboard flaps, even if strong, can be relaxed as a result of shearing displacement of the adhesive in the manner of cohesive failure, so that the adhesive can still hold the corrugated cardboard flaps in that displaced state.
On the other hand, adhesive tapes using pressure-sensitive adhesive compositions comprising the above described A-B-A type block copolymers as the base polymers, when applied for corrugated cardboard flap sealing under light pressure, cannot secure a sufficient adhering area because the cohesive strength the adhesive is too great. Furthermore, in such a case, the adhesive does not undergo cohesive failure-like displacement, so that the adhesive cannot resist the repulsive strength the corrugated cardboard flaps and is peeled from the corrugated cardboard surface in the manner of adhesive failure. The so-called hop-up state is encountered frequently.
For avoiding such disadvantage of the adhesive tapes using A-B-A type block copolymers, the possibility of using an A-B type block copolymer in combination with the A-B-A type block copolymer has been investigated. However, any satisfactory result has not been obtained to improve the corrugated cardboard sealing property (effect). In particular, this and other improving means in the prior art have generally the problem that the function of an adhesive to relax the stress of corrugated cardboard flaps due to cohesive failure-like displacement is contradictory to the function to hold the flaps.